Article such as jewelry or a wristwatch component having composite multi-film protective coating

ABSTRACT

The surface of a metal article which is exposed to the atmosphere and abrasion during normal use of the article is protected from scratches and/or corrosion during such use by a thin transparent abrasion-resistant film of an inert non-metallic material such as SiO 2 , SiC, Si 3  N 4 , TiO 2 , MgO, Al 2  O 3 , Ta 2  O 5 , Nb 2  O 5 , GeO 2 , spinel and selected colorless glass compositions. The protective film is preferably deposited by RF-sputtering techniques and undesirable coloration of the article by optical interference effects from incident light rays is avoided by properly correlating the film thickness with the refractive index of the particular material used to form the film. The invention permits the use of thinner gold plating on such items as articles of expensive jewelry and bracelets and cases for fine wristwatches without detracting from the quality, durability or appearance of the merchandise. Alternative embodiments in which several films of various selected non-metallic inert materials are combined to form composite protective coatings that provide additional cost and manufacturing advantages are also disclosed along with methods for sputter-depositing the protective films in the proper thicknesses to avoid optical discoloration effects, either on articles that have been previously plated with gold or which have been provided with a sputter-deposited layer of gold by sequentially operating the sputtering apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 612,492 filedMay 21, 1984 now abandoned, which is a division of application Ser. No.264,322 filed May 18, 1981, which application, in turn, is acontinuation-in-part of abandoned application Ser. No. 185,655 filedSept. 9, 1980.

BACKGROUND OF THE INVENTION

This invention generally relates to the art of protecting articles thathave metallic surfaces which are susceptible to abrasion damage orcorrosion and has particular reference to protecting gold-platedarticles of jewelry and wristwatch components (such as bracelets andcases) with a transparent coating of one or more selected inertmaterials. The invention also provides methods for coating such articleswith one or more overlying protective films that are transparent andcomposed of abrasion-resistant material.

As is well known, any articles of merchandise have metallic surfaceswhich inherently become dull or tarnished in the environment in whichthe article is used. For example, hardware items such asbuilding-identification plaques, handrails, doorknobs, decorativedoorknockers, etc. that are composed of brass or a similar metal oxidizequite rapidly and require constant polishing and waxing to maintain abrilliant pleasing appearance. This is also an age old problem witharticles such as flatware, trays, trophies, etc. that are made of silveror are silver plated.

While articles such as fine jewelry and the like that are made fromsolid gold or have gold-plated surfaces do not tarnish and thus do notpresent such a maintenance problem, they scratch easily and soon becomeunsightly when subjected to the constant abrasion and "rubbing action"encountered during normal everyday use. Since gold is a relatively softmaterial, it also wears away quite rapidly when subjected to suchconditions. If the article is gold-plated, this frequently exposes thebase metal and creates an unsightly corroded appearance in the case ofarticles (such as chains, rings, lockets, watchbands, etc.) that are indirect contact with the person's body. These characteristics thuspresent serious problems in the production and marketing of such itemsas gold-plated jewelry and gold-plated bracelets and cases forwristwatches. In order to compensate for the loss of gold that occursduring use by the customer, relatively thick gold plating is customarilyused on high quality merchandise of this type to ensure that the articlewill retain its original pleasing appearance. However, in view of theextremely high cost of gold and the likelihood that it will become evenmore expensive in the future, the use of such heavy gold platingpresents a serious economic problem in the watch and jewelry industries.

A practical and reliable means for protecting gold and gold platedarticles such as wristwatch components and the like from rapid wear andunsightly scratching (as well as corrosion if the gold plating has wornthrough) without materially changing its "natural" finish or appearancewould, accordingly, not only be very desirable from a quality andmarketing standpoint but would be very advantageous from a productionand cost reduction standpoint. Such protective means would also be veryuseful in preventing skin reactions and similar problems that aresometimes encountered by certain individuals when they wear a ring,chain or similar article that is made from a particular metal or alloy.

SUMMARY OF THE INVENTION

All of the foregoing objectives are achieved in accordance with thepresent invention by coating the metallic surface of the article with athin substantially transparent and colorless film of a selected inertand non-metallic material which tenaciously adheres to the surface andhas sufficient "hardness" to provide a very durable andabrasion-resistant protective finish and covering. In accordance with apreferred embodiment, the protective film is composed of a dielectrictype material such as silicon dioxide, magnesium oxide, aluminum oxide,titanium dioxide, spinel, silicon nitride, silicon carbide and varioustypes of glasses that have the proper combination of hardness,transparency and thermal expansion characteristics. Other dielectrictype materials which are substantially transparent in film thicknessesand are also suitable are tantalum oxide, niobium oxide, and germaniumoxide. Certain types of glasses can also be used as the protectivecovering or as "buffer" layers between the protective films and thesubstrates to compensate for differences in the thermal expansioncharacteristics of the substrate material and protective material.

The protective film of selected inert material is preferably depositedby RF-sputtering techniques and its thickness is controlled to preventundesirable discoloration of the article by optical interference effectsproduced by incident light rays which enter the film. Such interferenceeffects are exhibited by transparent films when the optical thickness(that is, the true thickness of the film multiplied by the refractiveindex of the film material) is comparable to the wavelength of light andthus falls within the range of from about 3,000 to 8,000 Angstroms (thevisible portion of the spectrum). Since the refractive index for thevarious film materials is different, the optimum thickness range willalso vary depending upon the particular material used to form theprotective film.

Composite type films which include one or more additional layers ofanother material are also employed in accordance with another embodimentof the invention to enhance the adhesion of the protective film as wellas the adhesion of a layer of a different metal that is sputtered ontothe substrate (as in the case of an article of base metal such as awatchband that is first coated with a sputtered layer of gold or anotherprecious metal). A composite coating consisting of very thin films of aprecious metal (such as gold) and interposed alternately-arrangedtransparent films of a protective material are employed in accordancewith another embodiment to further reduce the amount of precious metalrequired per article. Various methods of forming the protective filmsand also sequentially metal-coating and then protectively-coatingvarious articles composed of a base metal employingsputtering-deposition apparatus and techniques are also disclosed.

BRIEF DESCRIPTION OF THE DRAWING

A better understanding of the invention will be obtained from theexemplary embodiments shown in the accompanying drawing, wherein:

FIG. 1 is a plan view of a gold-plated band or bracelet for a wristwatchwhich has been protectively coated in accordance with the invention;

FIG. 2 is a fragmentary cross-sectional view, on a greatly enlargedscale, through a portion of the watch bracelet shown in FIG. 1 anddepicts the manner in which the thin plating of gold is protected by anoverlying transparent film of inert abrasion-resistant material;

FIG. 3 is a similar cross-sectional view of a conventional watchbracelet, on the same scale, and illustrates the much thicker goldplating commonly employed in the prior art for such watch components inthe absence of the protective coating means of the present invention;

FIG. 4 is a similar cross-sectional view of another embodiment whereinthe substrate is provided with an adhesion-promoting primer layer beforebeing coated with a sputtered layer of gold or the like and thenprotectively coated;

FIG. 5 is a fragmentary cross-sectional view on an enlarged scale ofstill another embodiment wherein a buffer or transition layer of aselected glass is employed between the transparent protective film andthe plated surface of the substrate to compensate for the difference inthe thermal expansion coefficients of the plated substrate andprotective film;

FIG. 6 is a similar view of an alternative embodiment in which twobuffer or transition layers of different glasses are employed;

FIG. 7 is a cross-sectional view of yet another embodiment wherein atransparent protective film of a selected inert material is depositeddirectly onto the unplated surface of a substrate or article that iscomposed of tarnishable metal;

FIG. 8 is a cross-sectional view of another embodiment of the inventionwherein alternating very thin films of a precious metal (such as gold)and a transparent protective material are employed to further reduce theamount of precious metal required to plate an article; and

FIG. 9 is a plan view of a case for a wristwatch which is respectivelyof other types of metallic articles that can be protectively coated inaccordance with the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention can be used with advantage to protectivelycoat various kinds of articles having metallic surfaces that are subjectto attack or corrosion by the environment in which they are used as wellas pieces of jewelry and the like that are plated with a layer of aprecious metal of a type which is easily scratched or rapidly worn awayduring normal use of the jewelry, it is especially adapted for use inconjunction with gold-plated articles such as bracelets and cases forwristwatches and the like and it has accordingly been so illustrated andwill be so described.

A representative watchband or bracelet 10 is shown in FIG. 1 andconsists of the usual intercoupled links L and a suitable latchingmember or clasp C. As illustrated in FIG. 2, such components aretypically fabricated from a suitable metal 12 (such as brass orstainless steel) which serves as a substrate for a plating 14 of gold, agold alloy, or other precious metal that provides the desired attractivelustrous finish. As is customary in the gold-plating art, a thin coating13 of nickel or other suitable metal is deposited on the substratebefore the plating operation is performed by electrode position or otherwell known means. Such an initial coating is referred to as a "strike"in the art and is required to insure that the gold plating bondsproperly to the substrate and that a smooth lustrous gold finish isproduced if the substrate has a rough surface. Nickel "strikes" on brasssubstrates typically have a thickness in the order of 0.10 micron (1,000Angstroms) or so. However, the thickness of such bonding layers is notcritical and can be varied according to the plating requirements and thecomposition and condition of the base metal.

In accordance with one of the important advantages afforded by thepresent invention, the thickness of the gold plating 14 is drasticallyreduced and the plated surface of the watch bracelet 10 (or otherarticle) is protected from scratching and abrasion by a film 16 of aselected inert and non-metallic material that tenaciously adheres to thegold-plated surface of the substrate 12. In order to provide adequatelong-term protection for the thin "soft" plating 14 of gold withoutaltering its appearance, the protective film 16 must be formed from amaterial which is much "harder" than gold and is substantiallytransparent and substantially colorless in thin film form.

Materials which meet all of these requirements and are thus suitable foruse as protective films in accordance with the invention are silicondioxide (SiO₂), aluminum oxide (Al₂ O₃), titanium dioxide (TiO₂),silicon nitride (Si₃ N₄), magnesium oxide (MgO), spinel MgO·3.5Al₂ O₃),Corning Glass No. 0080 (soda-lime glass), Corning Glass No. 7070,Corning Glass No. 7740 (PYREX glass), and Corning Glass No. 7059.

The various properties of these materials which make them suitable foruse as protective coatings pursuant to the invention are listed in TableI below. For comparison, electroplated gold has a Knoop hardness ofabout 130 and the thermal expansion coefficients for stainless steel andbrass are 173 and 169×10⁻⁷ /°C., respectively.

                  TABLE I                                                         ______________________________________                                        Protective                    Thermal Expansion                               Coating   Hardness  Refractive                                                                              Coefficient                                     Material  (Knoop)   Index (n) (× 10.sup.-7 /°C.)                 ______________________________________                                        SiO.sub.2 741       1.46       8                                              Al.sub.2 O.sub.3                                                                        1370      1.78      73                                              TiO.sub.2 879       2.71      96                                              Si.sub.3 N.sub.4                                                                        2500      1.87      45                                              MgO       692       1.69      120                                             Spinel    1140      1.73      59                                              Corning Glass                                                                           400        1.512    92                                              No. 0080  (approx.)                                                           Corning Glass                                                                           418        1.469    32                                              No. 7070  (approx.)                                                           Corning Glass                                                                           418        1.474    33                                              No. 7740                                                                      Corning Glass                                                                           424       1.53      46                                              No. 7059                                                                      ______________________________________                                    

The hardness data given in Table I is the Knoop microhardness for bulkmaterials and thus provides an indication of the abrasion resistance ofthe various materials and their ability to protect the underlyingplating of gold (or other precious metal).

Corning Glass No. 0080 is a soda-lime silicate type glass that is usedin the electric lamp industry for lamp bulbs and the like. Such glassestypically contain 60 to 75% (by wt.) SiO₂, 5 to 18% Na₂ O, 3 to 13% CaOor MgO (or a mixture thereof) and minor amounts of additional materialssuch as Al₂ O₃, and K₂ O. A specific example of a glass composition ofthis type is as follows: 73.6% SiO₂, 16% Na₂ O, 3.6% MgO, 5.2% CaO, 1%Al₂ O₃ and 0.6% K₂ O.

Corning Glass Nos. 7070 and 7740 are borosilicate type glasses thatcontain major amounts of SiO₂ and B₂ O₃ and various minor constituents.Glass No. 7740 is marketed by Corning under the trade name "Pyrex"glass. A specific example of a No. 7740 type glass is as follows: 80.5%(by wt.) SiO₂, 12.9% B₂ O₃, 2.2% Al₂ O₃, and 0.4% K₂ O.

A specific example of a No. 7070 type glass composition is as follows:70% (by wt.) SiO₂, 28% B₂ O₃, 1.2% Li₂ O, 1.1% Al₂ O₃, 0.2% MgO, 0.5% K₂O and 0.1% CaO.

Corning Glass No. 7059 is an aluminoborosilicate type glass whichtypically has the following composition; 50.2% (by wt.) SiO₂, 25.1% BaO,13.0% B₂ O₃, 10.7% Al₂ O₃ and 0.4% As₂ O₃.

Other dielectric type materials that are suitable for use as protectivefilms in accordance with the invention are silicon carbide (SiC) whichhas a Knoop hardness of 2500, tantalum oxide (Ta₂ O₅), niobium oxide(Nb₂ O₅) and germanium oxide (GeO₂). Another glass which has also beenfound suitable is a high-lead-content solder glass which has a thermalexpansion coefficient of 117×10⁻⁷ /°C. and typically contains 85% (bywt.) PbO, 7.5% B₂ O₃ and 7.5% SiO₂.

As indicated by the data given in Table I, the material which is used toform the protective film 16 should have a refractive index that is inthe range of from about 1.4 to about 2.8.

In general, any material that has a Knoop hardness of 400 or more, isinert, and has the ability to be deposited in thin adherent films thatare of controlled thickness and are also substantially transparent andcolorless in such thicknesses can be used as the protective coating. Ifthe thermal expansion characteristic of the protective material relativeto the substrate is such that flaking, cracking or peeling of the filmoccurs, then an intervening layer (or layers) of other materials must beused as hereinafter disclosed to correct the mismatch. Various types ofclear glass compositions (such as the aforesaid solder glass) that havethermal expansion coefficients which approximate that of stainlesssteel, for example, can also be used.

While the protective film 16 can be formed on the plated substrate 12 byvarious means including electron beam evaporation and chemicalvapor-deposition techniques, deposition by RF-sputtering is preferredbecause sputtered films, in general, exhibit excellent adhesion, aredense and free from pinholes, provide satisfactory substrate coverage,and have the proper stoichiometry.

As is well-known, transparent films will produce coloration due tooptical interference or so-called "Newton ring" effects when the opticalthickness (the product of the true or mechanical thickness and therefractive index) of the film is roughly of the same order of magnitudeas the wavelength of light (from about 3,000 to 8,000 Angstroms). Thefilm thickness range which enables incident light rays to produce suchinterference color effects thus depends upon the refractive index of thecoating material and generally lies between 0.05 micron (500 Angstroms)and 1.5 microns (15,000 Angstroms) for the materials which are listed inTable I or referred to as being suitable. Hence, in order to avoid suchundesirable discoloration of the gold-plated surface of the watchband 10(or other article which is being protectively coated), the thickness ofthe protective film 16 must either be less than about 500 Angstroms orgreater than about 15,000 Angstroms for these materials.

Since films with thicknesses less than 500 Angstroms would be too thinto provide adequate "long term" abrasion protection, protective filmsformed from the aforementioned materials must have thicknesses that aregreater than about 15,000 Angstroms. Protective films that are toothick, however, must also be avoided since they will tend to crack orpeel away from the substrate. Since the film thickness above whichoptical interference coloration effects are not discernible variesinversely with the refractive index of the coating material, a thinnerfilm of a high refractive index material can be employed. This isdesirable from a manufacturing standpoint since shorter film-depositiontimes will be required and peeling will be inhibited. The followingmaterials are preferred in this respect since they have high indices ofrefraction (shown in parenthesis): SiC (2.73), Nb₂ O₅ (2.24), Ta₂ O₅(2.21), TiO₂ (2.71), GeO (2.1) and Si₃ N₄ (1.87).

The sputtering yield provides a relative indication of how easily agiven material can be sputtered from a target and, hence, how rapidly aprotective film of that material can be formed by sputter-deposition.This is an important consideration when coating in mass-productionquantities is involved. In general, the ideal material for theprotective coating from both a quality and manufacturing standpoint isthus a material which has high values for hardness, refractive index,and sputtering yield.

TEST DATA AND SPECIFIC EXAMPLES

Preliminary tests performed on small stainless-steel plates which werecoated with a 2.5 microns (25,000 Angstroms) thick electroplated layerof 24 K gold have indicated that protective coatings of SiO₂, Al₂ O₃,TiO₂, Si₃ N₄ and SiC which were formed by RF-sputtering exhibitedexcellent adhesion and protection of the substrates against abrasion. Aportion of each of the gold-plated test plates was masked duringdeposition of the protective film to provide an uncoated "goldreference" surface for evaluation. Film adhesion was checked by aso-called "tape test" which consisted of firmly pressing a piece of"Scotch" brand adhesive tape onto a film-coated portion of the testplate and then stripping the tape away. This is a very demanding testsince any material which is not firmly bonded or securely anchored tothe substrate will be lifted from its surface when the adhesive tape isstripped away. Abrasion resistance was determined by vigorously rubbingthe coated and uncoated portions of the test plates with a pencil eraserand then with steel wool. The appearance of the coated portions of theplates was evaluated visually by noting any discoloration effects orundesirable altering of the natural color of the gold plating.

These preliminary tests verified that very thin films of the testedmaterials in the thickness range below that were required to avoidoptical interference effects (that is, less than about 0.05 micron or500 Angstroms) would not provide adequate "long-term" abrasionprotection of the gold-plated substrates. The sputtered films of SiC hada yellowish-brown tint and altered the natural gold color of the samplesto a certain degree. Such films would, accordingly, be satisfactory onlywhere a slight discoloration of the plated surface could be tolerated(or might even be desirable). Although the sputtered films of TiO₂ werecolorless and (due to their high refractive index) were devoid of anyoptical interference coloration even though they were only 1.5 micronsor 15,000 Angstroms thick, the high refractive index of this materialcaused the film surface to have a high reflectivity which tended to mutethe gold color of the substrate and slightly modify its appearance. TheAl₂ O₃ sputtered very slowly and would thus probably not lend itself tomass production operations.

Experiments have indicated that the sputtering yield of Si₃ N₄ iscomparable to that of SiO₂ (0.13 molecule per ion at 1 kilovolt targetvoltage) and these two materials are thus good selections forprotectively coating gold-plated substrates where coating times andcosts are critical factors.

Test data obtained with SiO₂ films of varying thickness on gold-platedstainless steel sample plates have shown that protective films of thismaterial that were approximately 1.4 microns or 14,000 Angstroms thickexhibited pale pink and green interference colors which indicated thatthe films were too thin. When films of SiO₂ 5 microns or 50,000Angstroms thick were deposited on such gold-plated sample substrates,the coatings cracked and flaked from the substrates due to high stressespresent within the thick films. SiO₂ films 3.4 microns (34,000Angstroms) thick adhered well to the gold-plated substrates and also hadexcellent abrasion resistance, on the basis of the "eraser and steelwool" test. The color of the coated plates was indistinguishable fromthe original gold-plated substrate. Pursuant to these experimental data,the optimum thickness range for SiO₂ protective films on gold-platedarticles is accordingly within the range of about 1.5 to about 4 microns(that is, from about 15,000 to about 40,000 Angstroms).

Similar tests conducted on gold-plated watch bracelets 10 of the typeshown in FIG. 1 confirmed the foregoing early test data obtained onplate samples. These additional tests revealed that cleanliness of thegold-plated substrates prior to deposition of the protective film isquite important. Some of the sample watchbands apparently had an organicfilm or coating on their surfaces which may have been intended to serveas a protective coating by the manufacturer. This contamination producedundesirable brown stains when SiO₂ protective films were applied andalso caused flaking of the films. These coating problems were solved bysubjecting the watchbands to a cleaning procedure which consisted ofboiling the components in a suitable detergent, rinsing them indeionized water and methyl alcohol, and then drying them in air ataround 120° C.

The additional series of tests also indicated that the optimum thicknessof SiO₂ protective films was somewhat less for the gold-platedwatchbands than for the gold-plated test blanks of metal. Films betweenabout 2 and 3.5 microns thick tended to flake from the watchbands and noundesirable interference color effects were produced if the filmthickness was greater than about 1.4 microns. The optimum thickness forprotective films of SiO₂ in the case of gold-plated watchbands of thekind shown in FIG. 1 is accordingly within the range of from about 1.4to about 2 microns (that is, from about 14,000 to about 20,000Angstroms).

The differences in the observed interference effects for SiO₂ filmsdeposited on the test blanks of gold-plated metal and on the gold-platedwatchbands may have been due to the smooth surface finish of the testblanks and the fact that the watchbands had a textured surface finish.

A most important advantage afforded by the present invention from a coststandpoint is the fact that wristwatch bracelets and other componentsfor fine watches (as well as various articles of fine jewelry) can beprovided with a much thinner plating of gold without detracting in anyway from the quality or appearance of the components or articles sincethe natural appearance of the gold plating is preserved by thetransparent film of protective material. As shown in FIGS. 1 and 2, awatchband 10 of high quality having a stainless steel substrate 12 whichis provided with a nickel "strike" 13 about 1,000 Angstroms thick andthen plated with a layer 14 of gold approximately 0.5 micron or 5,000Angstroms thick (dimension "t₁ ") can accordingly be made byprotectively coating the thin layer of gold with a film 16 of SiO₂ (orsimilar transparent material) that is approximately four times as thickas the gold plating--that is, a protective film of SiO₂ approximately 2microns or 20,000 Angstroms thick. The relative thicknesses of the goldplating, the bonding layer or "strike" and the protective film aresubstantially as shown in FIG. 2. Hence, the gold plating 14 is onlyone-fourth as thick as the protective film 16, and the nickel "strike"13, in turn, is only one-fifth as thick as the gold plating.

If the article or substrate is such that it can be coated with a thinnerlayer of gold which still retains the natural appearance and color ofsolid gold, then gold coatings in the order of about 0.2 or 0.3 micron(about 2,000 or 3,000 Angstroms) can be used in combination with theprotective film of the invention.

In contrast, conventional watch bracelets 11 (shown in FIG. 3) of goodquality having similar substrates 18 of stainless steel which is primedby a nickel "strike" 19 about 1,000 Angstroms thick generally have agold plating 20 that is approximately 10 microns or 100,000 Angstromsthick (dimension "t₂ ")--that is, twenty times thicker than the goldplating 14 employed in accordance with the invention. FIGS. 2 and 3 aredrawn to the same scale so that the relative thicknesses of the two goldplatings 14 and 20 is accurately shown in and apparent from the drawing.

Hence, the present invention permits the thickness of the gold platingsemployed on such fine watch components to be reduced by at least 95%(5,000 Angstroms versus 100,000 Angstroms) and up to 98% or so (2,000Angstroms versus 100,000 Angstroms) with a corresponding reductions inthe manufacturing cost of the watches. In view of the high cost of goldand other precious metals, the cost saving is very significant andconstitutes an important competitive advantage, not only in the watchindustry but in the manufacture of fine jewelry and similar articlesthat are composed of a base metal and presently require heavy coatingsor platings of a precious metal to preserve their appearance.

ALTERNATIVE "PRIMER LAYER" EMBODIMENT (FIG. 4)

Further tests on protective films for gold-plated articles such as watchbracelets have shown that an additional reduction in their manufacturingcost can be realized by depositing the gold layer on the watch braceletsby sputtering techniques (rather than electroplating) so that both thegold-coating and protective-coating operations can be performedsequentially within the vacuum chamber of the RF-sputtering apparatus.Experiments confirmed that this could be achieved quite readily byproviding one target of gold and another target of SiO₂ (or othermaterial from which the protective film is to be formed) within thevacuum chamber and then simply operating the sputtering apparatus in twodifferent modes which selectively bombarded the targets to first depositthe required layer of sputtered gold onto the watchbands of base metaland then coat the resulting gold-coated surface with a film of sputteredSiO₂, the required thicknesses being obtained by properly controllingthe target voltage, power input and the length of the sputteringoperation in each mode.

A further advantage afforded by this method of sequentially-coating wasthe ability to sputter-coat the watchbands with a layer of 24K goldinstead of the 14K gold conventionally used in the electroplatingprocess. The sputtered-gold layers thus had a deeper and richer goldcolor (due to the higher gold content) compared to watchbands withgold-electroplated coatings--even though the sputtered-gold layers weremuch thinner and used less gold.

During the course of these experiments, it was also discovered that boththe adherence and durability of the sputtered-gold coatings could beimproved by depositing a very thin layer of titanium (Ti) on thestainless steel substrate before sputter-depositing the gold layer. Theuse of a sputtered film of Ti about 200 Angstroms thick permitted alayer of sputtered gold less than 0.5 micron thick (5,000 Angstroms) topass the "tape test" for adhesion. Such a preliminary or "primer" layerof Ti accordingly enables sputtered gold layers approximately 0.25 or0.3 micron thick (2,500 or 3,000 Angstroms) to be employed onsubstrates--thus providing a corresponding further reduction in coatingand material cost without detracting from the appearance of the finishedarticle as regards its natural gold "finish" and appearance.

A watch bracelet 10a (or other article) having a substrate 12a of a basemetal (such as stainless steel or the like) that is provided with thecomposite sputtered coating according to this embodiment as shown inFIG. 4. As will be noted, the substrate 12a has a thin primer layer 21of titanium deposited on its surface to promote the adhesion of a layer22 of sputtered gold which, in turn, is protected by a film 16a of SiO₂or other suitable material that is substantially transparent and doesnot noticeably alter the natural appearance or finish of the gold layer.

The film thickness of the adhesion-promoting primer-layer 21 of titaniumis not especially critical and can be in the range of from about 50 to400 Angstroms. Suitable thin films of other metals such as chromium,nickel and Nichrome type alloys that have a similar adhesion-promotingeffect can also be used. Nichrome alloys are well known in the art andare composed of about 80% (by wt.) nickel and about 20% chromium.

The manufacture of the watch bracelet 10a or other article will also befacilitated if the primer layer 21 of titanium (or other metal) issputter-deposited on the substrate 12a in sequential fashion with theoverlying layers of gold and protective material in a common vacuumchamber of a properly modified and controlled RF-sputtering apparatus.

ALTERNATIVE "BUFFER LAYER" EMBODIMENT (FIG. 5)

Another form of composite protective coating for gold-plated watchcomponents and similar articles is shown in FIG. 5 and was developed toovercome an adherence problem encountered during trial runs using aproduction type sputtering system which operated at high depositionrates. When sputtering protective films of SiO₂ onto gold-platedstainless steel watchbands using such a system, it was discovered thatthe protective films sometimes flaked from the watchbands upon removalfrom the sputtering apparatus. While the exact cause of this problem isunknown, it is believed that it may be due to excessive heating of thewatchbands produced by the high rate at which the sputtered material wasdeposited--in combination with the subsequent cooling and a thermalexpansion coefficient mismatch between the SiO₂ film and stainless steelsubstrate which induced stresses in the films with resultant flaking.

It was found that this problem could be solved by using an additionltransparent layer of a suitable material between the SiO₂ film and thegold-plated stainless steel substrate, which additional layer iscomposed of a material that has a thermal expansion coefficient betweenthat of SiO₂ (8×10⁻⁷ /°C.) and that of stainless steel (173×10⁻⁷ /°C.).The additional layer thus serves as a buffer or "transition" layer thatcompensates for the expansion mismatch between the substrate andprotective SiO₂ film without interfering with the ability of the film toprotect and preserve the natural appearance of the gold plating.

The soda-lime silicate type glass marketed by the Corning Glass Companyunder the trade designation Corning Glass No. 0080 (listed in Table I)is a material particularly suitable for use as such a buffer layer sinceit has a thermal expansion coefficient of 92×10⁻⁷ /°C. (about midwaybetween that of stainless steel and SiO₂) and is transparent andcolorless in the thickness required. This glass composition also has anindex of refraction of 1.512 and a Knoop hardness of about 400 asindicated in the Table. However, any of the glasses listed in Table I(as well as the aforementioned solder glass) can be used as a buffermaterial since they all have thermal expansion coefficients that aremuch higher than that of SiO₂. Since materials such as TiO₂ and Al₂ O₃have high coefficients of themal expansion, they are also especiallysuited for use as a buffer material.

Tests have shown that stainless steel watchbands provided with a goldlayer (either plated or sputtered) approximately 5,000 Angstroms thickcan be provided with a sputtered composite transparent protectivecoating that exhibits excellent adhesion and durability and consists ofa layer of Corning No. 7059 Glass about 1.5 microns thick (15,000Angstroms) that was covered by a film of SiO₂ approximately 0.5 micron(5,000 Angstrom) thick. The relative thicknesses of the transition orbuffer layer of glass and the SiO₂ film are not critical and can varyconsiderably from the stated values. For example, the film thickness ofthe SiO₂ can be within the range of from about 0.2 micron to about 2microns (2,000 to 20,000 Angstroms) and the buffer layer of glass can befrom about 1 to 4 microns thick (10,000 to 40,000 Angstroms). However,the combined thicknesses of the protective film and buffer layer must besufficient to prevent the occurrence of optical interference effects andundesired coloration of the gold-coated surface of the article.

As illustrated in FIG. 5, a watchband 10b or other article according tothis embodiment consists of a substrate 12b of stainless steel (or otherbase metal), a "strike" or bonding layer 13b of nickel or the like, athin plating 14b of gold (or other precious metal) that is protectivelyshielded from scratching and other damage by a substantially transparentand colorless composite coating which consists of the buffer layer 24 ofa selected glass and a much thinner layer 25 of SiO₂ or other suitableabrasion-resistant material.

ALTERNATIVE MULTIPLE "BUFFER-LAYER" EMBODIMENT (FIG. 6)

The invention is not limited to the use of a single layer of a buffermaterial to correct the mismatch of the thermal expansion andcontraction characteristics of the gold-plated substrate and theprotective film but includes within its scope the use of two or morebuffer layers for this purpose. A multiple buffer-layer embodiment 10cis shown in FIG. 6.

As illustrated, this embodiment comprises a substrate 12c of a basemetal, the usual very thin "strike" or bonding layer 13c of nickel orthe like, a gold plating 14c of reduced thickness, two buffer layers 26,27 of two different substantially transparent colorless materials thathave thermal expansion coefficients which provide a "two-step"transition from the high expansion coefficient of the plated substrate12c to the much lower expansion of the transparent protective film 28.

In the case of a stainless steel gold-plated substrate and a protectivefilm of SiO₂, a first buffer layer of soda-lime silicate glass (No. 0080glass) and a second buffer layer of Corning No. 7059 Glass would be agood combination of buffer materials since they would provide a"balanced" transition from 173 to 92 to 46 to 8 (in terms of theexpansion coefficients of the respective materials, starting with thesubstrate).

The thickness of the buffer layers is not critical but they obviouslyshould be thin enough to maintain the composite coating below about40,000 Angstroms or so. They may be of equal thickness (as shown in FIG.6) or their relative thickness can be varied and correlated with theexpansion coefficients of the particular materials to provide an optimum"gradation" of stress forces at the interface of the substrate andprotective film. However, in FIG. 6 the total thickness of theillustrated composite coating (that is, buffer layers 26, 27 and theprotective film 28) is the same as the thickness of the compositecoating used in the single-buffer layer embodiment shown in FIG. 5. Thisis preferred since it would reduce the sputter-coating times to aminimum.

ALTERNATIVE SINGLE-COATING EMBODIMENT (FIG. 7)

The invention is also suitable for use in protectively coating articlesthat are entirely composed of a material (such as brass or silver) thatis rapidly degraded or becomes tarnished by chemical attack from oxygenor pollutants in the atmosphere in which they are used.

As shown in FIG. 7, in accordance with this embodiment the unplatedarticle 10d itself (a silver bowl or tray, or a brass name-plate, forexample) constitutes the substrate 12d that is provided with thesubstantially transparent and colorless protective film 30 of SiO₂ (orother suitable abrasion-resistant material such as those listed in TableI and in the text immediately following the Table). The thickness of thefilm 30, as in the previous embodiments, must be properly correlatedwith the refractive index of the particular protective material to avoidoptical interference effects and resultant undesirable coloration thatwould otherwise be produced by incident light rays. Since theaformentioned solder glass has a thermal expansion coefficient of117×10⁻⁷ /°C., it can also be used to form the protective film 30 inthose instances where the article is composed of a metal that also has ahigh expansion coefficient. For example, a protective film of this typeglass (or any of the other glasses listed in Table I) would be suitablefor use on silver trays, brass commemorative plaques or brass nameplatesused on buildings and the like, especially since such articles are notsubjected to severe abrasion during normal use.

If the protective film 30 is composed of a much harder material such asTiO₂, SiO₂, MgO or the like, then it can be used in conjunction withwatchbands or watch cases that are composed of solid brass and are thusnot coated or plated with gold or another precious metal. Experimentaltests have demonstrated that sputtered SiO₂ film adhere very well tobrass articles and produce a body color that is very similar to gold.SiO₂ films 2.58 microns thick (25,800 Angstroms) were free of blemishesand abrasion-resistant when sputter-deposited on a clean brasswatchcase.

ALTERNATIVE "MULTI-THIN-FILM " EMBODIMENT (FIG. 8)

Another form of composite layer that permits even thinner coatings andsmaller amounts of gold or other precious metals to be employed onsubstrates of a base metal is shown in FIG. 8.

According to this embodiment, an article 10e (such as a bracelet for awristwatch or a piece of jewelry) that is composed of a base metal (suchas stainless steel or the like that serves as a substrate 12e) is coatedwith a primer layer 13e of titanium or a similar material and then witha plurality of very thin films 31,33,35,37 of sputtered gold (or otherprecious metal) and a plurality of interposed thin films 32,34,36, 38 ofsputtered SiO₂ (or other transparent and inert protective material). Theseries of alternately-disposed sputter-deposited films of gold and SiO₂form a very hard and durable composite coating whose outer surfaceconsists of an SiO₂ film 38 and whose inner surface is a layer 31 ofgold that is bonded to the primer-coated substrate 12e. While a total ofeight interposed and overlapped films 31-38 are shown in FIG. 8, anysuitable or required number can be employed (as indicated by the"break-away" in the composite coating). Tests conducted with substrateshaving a total of ten such films gave satisfactory results.

The alternating films can be very thin (for example, less than about 100Angstroms thick) and provide several advantages in that they greatlyreduce the sputtering times (and thus the overall coating cost) butstill produce an article 10e with a finish that has the natural look ofgold but has excellent abrasion-resistant properties and actuallycontains a very small total amount of gold metal. As the top films "wearaway" due to their extreme thinness, the next layer (of gold or SiO₂)provide the desired gold "finish" appearance.

The total amount of gold in such multi-film composite coatings can befurther reduced by making the gold films thinner than the films ofprotective material--for example, gold films that are around 50Angstroms thick in combination with SiO₂ films about 100 Angstromsthick. Hence, a composite coating having a total of forty such filmswould have an overall thickness of only 3,000 Angstroms (with theaggregate or "total" film thickness of the gold films being only 1,000Angstroms and thus requiring a very small quantity of gold).

Since the interposed protective films are so thin, they do not produceany optical interference effects or discoloration of the underlying goldfilms and can be sputter-deposited very rapidly. Flaking or peeling ofthe films is also not a problem since the sputtered materials areintimately bonded to one another and are not thick or brittle enough tocreate stresses due to mismatches of thermal expansion coefficients,either with respect to the interposed films themselves or with respectto the basemetal substrate.

While the combination of interposed films of SiO₂ and gold have beenreferred to in the embodiment just described, it will be apparent tothose skilled in the art that various other combinations of materialscan be used--depending upon the type of article involved (for example,alternating films of silver and spinel or a suitable glass, alternatingfilms of platinum and TiO₂ or MgO, etc.).

WATCHCASE EMBODIMENT (FIG. 9)

The invention is not limited to protectively coating bracelets or bandsfor wristwatches but includes within its scope the provision oftransparent abrasion-resistant coatings (consisting of one or severalfilms) on other components for wristwatches such as watchcases 40 of thetype shown in FIG. 9. Such cases can either comprise a gold-plated basemetal (such as stainless steel or brass) or they can be composed of ametal such as brass that is not plated with gold but has a body colorwhich is very similar to gold.

The various protective films and composite protective coatings of thepresent invention can accordingly be used on any article of manufacturethat has a metallic surface which is degraded or tarnished by chemicalattack from the atmosphere or from pollutants in the atmosphere in whichthe article is used. The films and coating can also be used on articlessuch as jewelry and decorative components which have a substrate that iscomposed of a base metal (such as stainless steel or the like) that iscoated with a relatively soft metal (such as gold) which is easlyscratched and has poor "wearing" characteristics.

Another more limited but important benefit afforded by the protectivefilms and coatings of the present invention is in the prevention of"allergic" type reactions which some persons experience when wearingrings or chains, etc., that are composed of a certain metal or alloy.Since all of the materials listed are chemically inert and stable, a tinfilm of such material physically isolates the wearer's skin from thereaction-causing metal and thus permits the ring or other article to beworn without any undesirable biological effects or reactions. This isespecially true in the case of the glass or glass-like film-formingmaterials listed or mentioned previously.

SPECIFIC EXAMPLE OF SPUTTER-COATING PROCESS

Following is a specific example of the manner in which the substantiallytransparent abrasion-resistant films or coatings of the presentinvention are applied to articles or substrates by sputter-depositionusing experimental apparatus.

The articles or substrates are loaded into an RF-sputtering apparatusalong with a suitable target of the selected coating material, a 6-inchdiameter target of SiO₂ for example. The sputtering chamber is thenevacuated to a pressure of about 5×10⁻⁷ Torr and filled withapproximately 1.4×10⁻² Torr of a mixture of 90% argon and 10% oxygenwhich serves as the sputtering gas. An RF target voltage of 750 volts isthen applied to the target so that the power input is around 200 watts.The sputtering apparatus was operated for approximately 10 hours underthese conditions and films of SiO₂ having a thickness of from 1.5 to 2.0microns (15,000 to 20,000 Angstroms) were deposited on the substrate. Ifa production type magnetron RF-sputtering apparatus were used, the timerequired to deposit SiO₂ films of such thicknesses could be reduced toapproximately 1 or 2 hours.

As will be appreciated to those skilled in the art, the operatingparameters of the sputtering apparatus can be adjusted in accordancewith the sputtering yields, etc., of the various coating materials sothat films of the thicknesses required for each of the describedembodiments can be readily and efficiently deposited.

ADDITIONAL SPECIFIC EXAMPLES OF VARIOUS EMBODIMENTS

In addition to the experimental and test samples and data describedpreviously, following are specific examples of additional embodimentsthat have been made and evaluated:

FIG. 4 Embodiment--a stainless steel watchband was first coated with aprimer layer of Ti 260 Angstroms thick which was sputter-deposited in achamber evacuated to a pressure of 4×10⁻⁷ Torr and then filled withargon (the sputtering gas) to a pressure of 1.4×10⁻² Torr. The Ti filmwas deposited in approximately eleven minutes using an RF target voltageof 950 volts and a power input of around 200 watts. A gold film 4,900Angstroms thick was then sputter-deposited (without removing thewatchband from the chamber) by operating the sputtering apparatus fortwenty minutes (with a 24K gold target) at a target voltage of 750 voltsand 100 watts power input. A proective film of SiO₂ approximately 20,000Angstroms thick was then sputter-deposited by operating the apparatus ata target voltage of 750 volts and 200 watt power input for ten hours(using an SiO₂ target). The coatings adhered very well to the substrateand the SiO₂ film was colorless and passed the abrasion and adhesiontests described previously.

FIG. 5 Embodiment--A stainless steel watchband, which was previouslyelectroplated with gold in the conventional manner, was provided with abuffer layer of Corning No. 7059 glass that was 1.5 microns (15,000Angstroms) thick and then coated with a protective film of SiO₂ 0.5micron (5,000 Angstroms) thick by operating an RF-sputtering apparatusin sequential fashion with two different targets. The sputtering chamberwas first evacuated to a pressure of 6×10⁻⁷ Torr and then filled with asputtering gas consisting of 90% argon and 10% oxygen at a pressure of1.4×10⁻² Torr. The buffer layer of glass was deposited by operating theapparatus at a target voltage of 450 volts and a power input of 300watts for four and one-half hours, and the SiO₂ film was deposited byusing a target voltage of 750 volts and operating the apparatus forthree hours at 200 watts input.

FIG. 7 Embodiment--A brass watchcase was coated with a transparentcolorless protective film of SiO₂ 2.58 microns (25,800 Angstroms) thickby first evacuating the sputtering chamber to a pressure of 5×10⁻⁷ Torr,then filling it with a sputtering gas consisting of 90% argon and 10%oxygen at a pressure of 1.4×10⁻² Torr and operating the apparatus forfifteen hours at 200 watts input and 750 volts applied to the SiO₂target. The SiO₂ film adhered well, had no visual defects and passed theaforementioned "tape" and abrasion tests.

As will be apparent to those skilled in the art, when the previous metalcoating or plating on the article consists of gold, it need not becomposed of pure (24K) gold but can comprise a suitable gold alloy (forexample, 10K or 14K gold, etc.).

I claim as my invention:
 1. An article that is composed of a base metalat least a portion whereof is covered by a composite coating comprisingessentially a series of overlapped alternately-arranged thin films of(a) a metal that is different from the base metal and (b) a selectednon-metallic material which is substantially transparent and ofsufficient hardness to constitute an abrasion-resistant film,the firstfilm in the series comprising a film of said metal that is bonded to thebase metal, and the last film in the series comprising anabrasion-resistant substantially transparent film of said non-metallicmaterial which thus constitutes the outer surface of the coated portionof the article.
 2. The coated article of claim 1 wherein;said thin filmsof metal are composed of a precious metal, and said thin substantiallytransparent abrasion-resistant films are composed of a non-metallicinert material of the group consisting essentially of SiO₂, SiC, Si₃ N₄,TiO₂, Al₂ O₃ MgO, Ta₂ O₅, Nb₂ O₅, GeO₂, spinel, and selected glasses. 3.The coated article of claim 1 wherein;the thin substantially transparentabrasion-resistant films are composed of SiO₂, the thin films of metalare composed of gold or a gold alloy, and each of the films of SiO₂ andor gold alloy have a thickness less than about 100 Angstroms.
 4. Thecoated article of claim 1 wherein said thin films of metal are composedof a precious metal selected from the group consisting of gold, a goldalloy, silver and platinum.
 5. The coated article of claim 4 whereineach of the precious metal films is thinner than the respectiveabrasion-resistant films.
 6. The coated article of claim 4 wherein saidbase metal comprises stainless steel or brass and the article comprisesa piece of jewelry, a watch bracelet or a case component for awristwatch.
 7. The coated article of claim 4 wherein;the substantiallytransparent, abrasion-resistant films are also substantially colorlessand of substantially uniform thickness, and said precious metal filmsare also of substantially uniform thickness.
 8. The coated article ofclaim 7 wherein the thickness of each of the precious metal films isabout 50 Angstroms and the thickness of each of the abrasion-resistantfilms is from about 50 to 100 Angstroms.
 9. The coated article of claim2 wherein the first film of precious metal is bonded to the base metalby a layer of primer material composed of a selected metal.
 10. Thecoated article of claim 9 wherein said primer material is a metal of thegroup consisting of titanium, chromium, nickel and a Nichrome alloy. 11.The coated article of claim 10 wherein;said base metal is brass orstainless steel, and said layer of primer metal has a thickness of fromabout 50 to 400 Angstroms.
 12. The coated article of claim 4 whereinsaid article comprises a piece of jewelry, a watch bracelet, or a casecomponent for a wristwatch.
 13. The jewelry piece, watch bracelet orwristwatch case component of claim 12 wherein said alternately-arrangedthin films of precious metal and abrasion-resistant material comprisefilms of sputtered precious metal and said material.
 14. The jewelrypiece, watch bracelet or wristwatch case component of claim 12 whereinthe composite coating comprises alternately-arranged thin films of goldor a gold alloy in combination with films of SiO₂, films of silver incombination with films of spinel or a selected glass, or films ofplatinum in combination with films of TiO₂ or MgO.
 15. The jewelrypiece, watch bracelet or wristwatch case component of claim 12 whereinthe total number of said alternately-arranged and overlapped thin filmsranges from about 4 to 40.